[0001] The object of this invention is a sealing ring moving on sliding and/or rotating
units which is particularly suitable for use at extremely low temperatures, according
to the preamble of claim 1. Such type of sealing rings are known from DE-C-3 721 967.
[0002] For installations of various types operating at high temperatures, particularly in
steam power-stations and the like, there have hitherto been widely used sealing devices
acting on the surface of rotating or sliding components, made with an asbestos base
which ensured the necessary characteristics of resistance.
[0003] In particular with regard to pistons, for example for the operation of automatic
valves, actuated by a process fluid such as water or steam up to temperatures of 550°C
and pressures of 250 bar, the seals are comprised of sliding rings which are subjected
to severe mechanical stress.
[0004] Following recognition of the hazardous features of asbestos for the health of operators
there has been imposed a ban on the use thereof, thus creating the problem of providing
sliding sealing devices, usually in the form of rings, imparting a sliding seal against
the relevant internal or external cylindrical surface and furthermore, if required,
a front stationary seal made of non-asbestos material.
[0005] To this end there have been used materials of various types, but it has been found
that the products currently available on the market are not suitable for the appointed
purpose.
[0006] There have also been used experimentally sealing devices made of graphite, this being
a material which can withstand fairly well the aforesaid operating conditions and
imparts an optimum seal, but it is of extremely low mechanical strength and tends
to flake and slip, particularly if exposed to a high-velocity flow.
[0007] There are also known special forms of graphite rings capable of withstanding the
high temperature and pressure conditions described above.
[0008] All these proposed solutions, however, display serious shortcomings when the working
conditions are characterized on the contrary by an extremely low temperature, as in
the case of transport by road or rail by means of tankers for the conveyance of hazardous
substances such as flammable and/or lethal fluids which, in addition to their self-cooling
features, may encounter outside temperatures well below 0°, in particular lower than
minus 15°C.
[0009] Under such conditions, the materials used for manufacturing the seats accommodating
the sealing rings, as well as the rings themselves, tend to shrink rather than expand
as is the case with high temperatures, each according to its own coefficient of expansion,
causing enlargement of the seals resulting in leakage of fluid.
[0010] The same effect occurs during periods of temporary contact of the fluid at low temperature
with the actual seals, inasmuch as the rapid albeit temporary drop in temperature
may cause differential shrinkage of the materials as indicated above.
[0011] There is therefore posed the technical problem of providing a sealing device which
is particularly suitable for operating under temperature conditions well below zero
and furthermore displays adequate durability, exerting resistance both from a chemical
and thermal viewpoint and from a mechanical viewpoint, even in the presence of dirty
fluids in which there may be encountered inclusions of solid and abrasive substances,
and which is moreover compatible with the generality of the materials normally used
to make piston valves and the like.
[0012] Within the scope of the aforesaid problem, a further object of this invention is
to provide a seal capable of being used indiscriminately with the most widely-varying
materials in order to obviate the need for complicated choices to render compatible
the connection between the sealing components and the materials used for manufacturing
the locating seats, for example of valves, thus making it possible to eliminate the
dimensional constraints intended to lessen the likelihood of seizure and also to reduce
the manufacturing range, and therefore the range of stocks needed by the manufacturer
and by the distributor in order to cater for various applications.
[0013] Such results are achieved by means of the present invention, which provides a sealing
ring for sliding units operating at low or high temperatures and pressures.
[0014] Further details may be obtained from the following description with reference to
the attached drawings, which show:
In figure 1 : a sealing device fitted between a moving shaft and a fixed seat, according
to a partial schematic cross-section;
In figure 2 : an enlarged detail cross-section of the seal according to the invention;
In figure 3 : the seal assembly according to the invention applied to a smooth barrel
(not shown).
[0015] As illustrated in detail in figure 1, the seal assembly according to the invention
is comprised of a pair of rings 1 separated by a spacer 2 and secured as an assembly
to the bottom of seat 3 by a retainer 4 or the like; within such rings there slides
a shaft 5 of a piston or the like of a valve body 6.
[0016] Each ring 1 illustrated in enlarged cross-section in figure 2 is formed of annular
layers of graphite 1a and metal supporting plates 1b alternating with one another
so that the two opposite outer layers always consist of graphite 1a.
[0017] Metal supporting plate 1b, which may for assembly purposes only be made integral
with graphite layer 1a by means of adhesives or the like, is preferably made of stainless
or nickel steel and is of low thickness, whilst the thickness of graphite layer 1a
is several times greater than that of metal plate 1b.
[0018] The function of the metal supporting plate is to provide sufficient support so as
to facilitate the process of shaping rings 1, without impairing the sealing performance
required of the graphite itself; it is however also possible to use pure, non-reinforced
graphite, in cases where its brittleness is not an influencing factor.
[0019] Metal supporting rings 1b are conveniently made of a material withstanding process
fluids; the inside diameter of the rings, that is to say towards the sliding member,
is slighter greater than that of graphite rings 1a, so that contact with the sliding
surface of piston 5 is only made by such graphite rings 1a.
[0020] The adhesive used to hold together the graphite rings and the supporting rings does
not have any special features and does not need to be capable of imparting resistance
under the process conditions for which the sealing ring is provided; this does not
however affect the efficient performance of the sealing ring, because the adhesive
completes its task when the ring has been fitted to its seat; from that moment on
the ring as a whole is secured to the housing and there is no longer any likelihood
of its walls crumbling.
[0021] Sealing rings 1 each comprise a number of graphite rings 1a exceeding two in relation
to the diameter and overall thickness of such sealing ring; the distance between two
consecutive supporting rings, that is to say the thickness of the graphite layer,
ranges from 0.5 mm to a few millimetres; the thickness of the opposite layers of graphite
1a towards the outside of the ring is preferably about 0.5ö1.5 mm and not greater
than 2 mm.
[0022] The restriction concerning the thickness of graphite of the rings is intended to
avoid having an excessive thickness exposed to the mechanical action of the process
fluid and of the sliding members in contact with the actual sealing rings, which would
bring about deterioration of the graphite.
[0023] Such arrangement, which provides for the presence of two graphite layers 1a on the
head surfaces of the sealing ring, is suited to the case in which, as shown in figure
1, ring 1 is required to exert sealing action not only on the internal and external
cylindrical surfaces, marked S₁ in the figure, but also on front surfaces S₂ and S₃:
in this case sealing action on the front surfaces is provided by graphite layers 1a.
[0024] Figure 3 shows the application of sealing rings 1 to a piston 7 designed to slide
within a cylindrical barrel in order to operate as an actuator or the like.
[0025] According to such design, piston 7 integral with shaft 5 is provided with two pairs
of sealing rings 1 of the type described above, separated by spacer 2c, clamped by
plate 8 secured via screws 9 to spacer 2 of piston 7.
[0026] In order to impart greater rigidity to the assembly, between rings 1 it is preferable
to interpose a secondary supporting ring, not illustrated, of sufficient thickness
in relation to the outside diameter of the piston so as to achieve rigid containment
of rings 1. It is therefore obvious from the description provided above that the sealing
rings according to the invention ensure improved performance features compared with
conventional rings made of asbestos, which are no longer allowed to be used, displaying
a lower coefficient of friction, greater durability and greater dimensional stability,
particularly under operating conditions which provide for very low temperatures, without
thereby bringing about sealing defects.
1. Ein Dichtungsring (1) für Gleiteinheiten (3, 7), die sowohl bei sehr geringen Temperaturen
als auch bei hohen Drücken arbeiten, wobei der Dichtungsring eine Mehrzahl ringförmiger
Lagen umfaßt,
dadurch gekennzeichnet, daß die Lagen aus mindestens einem Paar Graphitringen (1a) bestehen, die sich mit
mindestens einem dazwischenliegenden Metallstützring (1b) abwechseln, wobei die Graphitringe
die äußeren Lagen des Dichtungsrings bilden und wobei die Dicke der Graphitlagen zwischen
0,5 bis 1,5 mm liegt und die Dicke der Metallplatte zwischen 0,01 mm und 0,03 mm liegt,
so daß der Kontakt mit der Dichtungsoberfläche durch die Graphitringe (1a) hergestellt
wird.